1. Field of Invention
This invention relates to plasma sources, primarily for processing workpieces such as semiconductor wafers or flat panel displays.
2. Description of Prior Art
Plasma sources are used to generate ionized gas plasmas to provide energy for modifying the surface of workpieces such as semiconductor wafers or flat panel displays. These surface modifications include etching selected portions of the surface, chemically modifying the surface, cleaning the surface, and depositing materials on the surface. Critical characteristics of plasma sources include controllable plasma density, controllable plasma uniformity, direction and magnitude control of ion kinetic energy, and minimum damage to either the workpiece or the process chamber.
Plasma sources are also used to provide ions for high energy ion implantation below the surfaces of workpieces.
Ogle, in U.S. Pat. No. 4,948,458, METHOD AND APPARATUS FOR PRODUCING MAGNETICALLY-COUPLED PLANAR PLASMA, describes the first high density plasma source generating a planar plasma conforming to the shape of a semiconductor wafer, by driving radio frequency current through a planar spiral coil to couple magnetic energy through a window into free electrons in the process gases by transformer action. While a dramatic step forward, this source does not provide easy means of optimizing uniformity, and wastefully generates a magnetic field above the coil, where it is not useful.
Ogle, in U.S. Pat. No. 5,277,751, METHOD AND APPARATUS FOR PRODUCING LOW PRESSURE PLANAR PLASMA USING A COIL WITH ITS AXIS PARALLEL TO THE SURFACE OF A COUPLING WINDOW, describes a planar plasma source using a coil with its axis parallel to the workpiece surface to generate a planar plasma. This plasma source accelerates free electrons in the process gas in a linear motion, rather than in a circular motion, as in U.S. Pat. No. 4,948,458.
Ogle, in U.S. Pat. No. 5,435,881, APPARATUS FOR PRODUCING A PLANAR PLASMA USING VARYING MAGNETIC POLES, describes a planar plasma source using spaced opposite polarity varying magnetic poles to generate a planar plasma.
Ogle, in U.S. Pat. No. 5,944,942, VARYING MAGNETIC POLES SOURCE, describes improvements in varying magnetic pole plasma sources, including the use of a metal structure with small oval windows in place of the large insulating window used with the earlier planar plasma sources. When used with ferromagnetic cores this plasma source greatly reduces the wasted magnetic flux away from the process chamber. In addition, adjusting the vertical positions of the ferromagnetic cores provides an easy method of optimizing uniformity, and the metal structure provides an effective voltage reference plane.
While plasma sources using spaced varying magnetic poles provide the optimum planar plasma, they require a relatively complicated structure to support and connect the coils and ferromagnetic cores.
Ogle, in U.S. Pat. No. 5,994,236, PLASMA SOURCE WITH PROCESS NONUNIFORMITY IMPROVED USING FERROMAGNETIC CORES, describes the use of ferromagnetic cores with magnetically coupled plasma sources to improve process nonuniformity and increase efficiency.
Jeng et al, in U.S. Pat. No. 6,462,483, INDUCTION PLASMA PROCESSING CHAMBER, describes a solenoidal coil with its axis perpendicular to a coupling window to couple varying magnetic flux into a process chamber. This coil arrangement concentrates the varying magnetic flux adjacent to the coil turns, with relatively little varying magnetic flux near the center of the coil. In addition, most of the magnetic field energy is stored in the volume above the coupling window, and so does not contribute to plasma generation inside the chamber. The coil inductance can be greatly affected by the conductive support structure of the chamber. In particular, if there is a conductive aperture below the coil with a diameter only slightly larger than the coil diameter, the inductance of the coil is significantly reduced, resulting in large currents for a given radio frequency magnetic flux coupled into the chamber.